1. Field of the Invention
The present invention relates to a preparation process for acrylic acid ester polymers. More specifically, the present invention pertains to a preparation process of acrylic acid ester polymers including acrylic acid ester homopolymers and block copolymers having at least one polymer block comprising an acrylic acid ester.
2. Description of the Background
On the anionic polymerization of an acrylic acid ester, the following reports (1) and (2) have been made.
(1) An acrylic acid ester homopolymer is available by the solution polymerization of an acrylic acid ester under low-temperature conditions of -78.degree. C. or so in a mixed solvent of toluene and tetrahydrofuran by using an organolithium compound as a polymerization initiator and lithium 2-(2-methoxyethoxy)ethoxide as an additive. A block copolymer having an acrylic acid ester polymer block and a methyl methacrylate polymer block is available by successively carrying out polymerization of an acrylic acid ester and polymerization of methyl methacrylate under the polymerization conditions similar to the above. (See Macromolecules, 27, 4890-4895(1994); Macromolecules, 27, 4908-4913(1994); Journal of Polymer Science: Part A: Polymer Chemistry, 35, 361-369(1997), et al.)
(2) An acrylic acid ester homopolymer is available by the solution polymerization of an acrylic acid ester in toluene under temperature conditions of -60.degree. C. or -78.degree. C. by using t-butyl lithium as a polymerization initiator and methylbis(2,6-di-t-butylphenoxy)aluminum or ethylbis(2,6-di-t-butylphenoxy)aluminum as an additive. A block copolymer having an acrylic acid ester polymer block and a methacrylic acid ester polymer block is available by successively or simultaneously carrying out polymerization of an acrylic acid ester and polymerization of a methacrylic acid ester under the polymerization conditions similar to the above. (See Polymer Preprints, Japan, 46(7), 1081-1082(1997) and ibid, 47(2), 179(1998).)
Anionic polymerization processes which, however, do not relate to an acrylic acid ester but a methacrylic acid ester have been reported as described below in (3) to (6).
(3) Poly(methyl methacrylate) having at least 80% of syndiotacticity is formed by the solution polymerization of methyl methacrylate in toluene by using t-butyl lithium as a polymerization initiator and a trialkyl aluminum as an additive. (See Makromol. Chem., Supplement, 15, 167-185(1989).)
(4) Poly(methyl methacrylate) having syndiotacticity of about 70% is formed by the solution polymerization of methyl methacrylate in toluene in the presence of diisobutyl (2,6-di-t-butyl-4-methylphenoxy)aluminum by using t-butyl lithium as a polymerization initiator. (See Macromolecules, 25, 5907-5913(1992).)
(5) A methacrylic acid ester polymer having a narrow molecular weight distribution is formed by polymerizing a methacrylic acid ester at a temperature range of from -20.degree. C. to +60.degree. C. by using an organic alkali metal compound such as t-butyl lithium as an initiator and a specific organoaluminum compound having at least one bulky group (for example, triisobutylaluminum, diisobutyl(2,6-di-t-butyl-4-methylphenoxy)aluminum, or the like) as an additive. This polymerization process can be applied to the preparation of a block copolymer. (See, U.S. Pat. No. 5,180,799.)
(6) Poly(methyl methacrylate) having an at least 70% content of syndiotactic triads can be obtained by mixing an organolithium initiator with a ligand such as methylbis (2,6-di-t-butylphenoxy)aluminum, ethylbis(2,6-di-t-butylphenoxy)aluminum, isobutylbis (2,6-di-t-butylphenoxy)aluminum or tris(2,6-di-t-butylphenoxy)aluminum at a temperature of at least 0.degree. C. and then adding methyl methacrylate to the resulting mixture to anionically polymerize said methyl methacrylate. This process is applicable to the preparation of a block copolymer having a polymer block composed of methyl methacrylate and another polymer block composed of a monomer selected from another methacrylic monomer, aromatic vinyl monomer, diene and maleimide. (See U.S. Pat. No. 5,656,704.)
According to the U.S. Pat. No. 5,180,799 described above in (5), the polymerization reaction is suppressed when the anionic polymerization process of a methacrylic acid ester as described in the patent in the presence of an organoaluminum compound having a bulky group is applied to an acrylic hydrogen atom-containing monomer.
Upon anionic polymerization of a monomer on an industrial scale, it is not completely avoidable that a polymerization initiator to be used has already been partially deactivated and the deactivation of the polymerization initiator proceeds further in the polymerization system owing to the impurities, such as water, which are introduced into the polymerization system as a mixture with the monomer, solvent or the like. Accordingly, it is impossible to prepare a polymer having a desired molecular weight with good reproducibility even by carrying out anionic polymerization using stoichiometric amounts of a polymerization initiator and a monomer calculated based on the desired molecular weight of the polymer. When the living properties in anionic polymerization are high, in other words, when a living polymer with a long-life active anionic end is formed in the reaction system, a polymer having a desired molecular weight and a narrow molecular weight distribution can be prepared with good reproducibility by polymerizing a monomer in an amount smaller than the stoichiometric amount calculated based on the amount of a polymerization initiator used, thereby forming a living polymer; and polymerizing the additional amount of the monomer with the living polymer, after measuring the molecular weight of the living polymer, calculating the number of moles of the living polymer based on the molecular weight of the living polymer and amount of the monomer, and calculating the additional amount of the monomer based on the molecular weight and the number of moles of the living polymer and the desired molecular weight of the final polymer. When the living properties are low in the anionic polymerization, on the other hand, even if a two-stage polymerization technique as described above is employed, a polymer available by the second stage polymerization inevitably has both a component lower in molecular weight and a component higher in molecular weight, than that desired and therefore has a wide molecular weight distribution. This results from the time-dependent marked deactivation of anions at the end of the polymer obtained by the first polymerization. Low living properties in the anionic polymerization are accompanied by deactivation which proceeds even during the polymerization reaction so that at a relatively low polymerization rate, the molecular weight distribution of the resulting polymer inevitably becomes wide even by the first-stage polymerizing operation.
For the preparation of a block copolymer by anionic polymerization, a technique of polymerizing a certain monomer to form its living polymer and then adding another monomer to the polymerization system tends to be adopted. Also in this case, the living properties have a large influence on the block formation efficiency.
Since the anionic polymerization of a monomer such as an acrylic acid ester is an exothermic reaction, so that when such anionic polymerization is carried out under cooling conditions in an industrial scale, it becomes very important to control the temperature rise in the polymerization system due to the exothermic heat. With a view to overcoming this problem, a technique of continuously feeding a monomer to the reaction system at a predetermined rate, thereby controlling the polymerization rate is sometimes adopted. When polymerization is conducted by continuously feeding a monomer, however, living properties in the polymerization reaction tend to have an influence on the uniformity of the molecular weight distribution of the resulting polymer. In other words, not so high living properties inevitably widen the molecular weight distribution of the resulting polymer.
Also in the case where after anionic polymerization, the resulting polymer having an active anionic end is reacted with a functionalizing agent to prepare a polymer having at an end thereof a functional group, high living properties are required for heightening the introduction ratio of a functional group.
In the preparation process of an acrylic acid ester polymer as described in (1), use of tetrahydrofuran as a part of a solvent is necessary in order to polymerize an acrylic acid ester with high living properties, thereby obtaining the desired polymer in a high yield. It is however not easy to industrially use tetrahydrofuran or collect and purify tetrahydrofuran at high purity because of its water absorption properties or mixing with peroxides. When a primary alkyl acrylate such as n-butyl acrylate is used as an acrylic acid ester, polymerization at markedly low temperature conditions, as low as about -80.degree. C., is required in order to attain high living properties. It is difficult to industrially apply this process, because, as described above, it needs a solvent which is not suitable for use in bulk judging from its handling properties and in addition, large cooling utilities must be used.
In the preparation process of an acrylic acid ester polymer described in (2), it takes a long time to complete polymerization when the temperature of the polymerization system is markedly low, about -78.degree. C., since the polymerization rate is low. When the temperature of the polymerization system is relatively high, -60.degree. C. or higher, on the other hand, deterioration in the living properties occurs, which widens the molecular weight distribution of the resulting polymer or makes it difficult to control the molecular weight by the two-stage polymerization process as described above. When a block copolymer is prepared as an acrylic acid ester polymer, the anionic end of the growing species has an insufficiently long life, so that the block formation efficiency is insufficient and it is difficult to prepare a high-purity block copolymer having a narrow molecular weight distribution on an industrial scale. The process of (2) is therefore industrially disadvantageous.
In the case of anionic polymerization of a methacrylic acid ester in the presence of an initiator system composed of an organic alkali metal compound and an organoaluminum compound, as is adopted in the above-described processes (3) to (6), since the addition of the organoaluminum compound is effective for suppressing the side reaction of the anion of the growing species against the ester group of the methacrylic acid ester monomer, a methacrylic acid ester polymer having a narrow molecular weight distribution can be obtained. In the above-described U.S. Pat. No. 5,180,799, it is described that the polymerizing reaction is suppressed when the anionic polymerization process of a methacrylic acid ester as described in the above patent specification is applied to an acrylic hydrogen atom-containing monomer. The present inventors tried to apply some of the anionic polymerization processes of a methacrylic acid ester as described in (3) to (6) to the polymerization of an acrylic acid ester, which however did not bring about good results. The polymerization of a primary alkyl acrylate was found to be particularly difficult. These results are presumed to owe to the side reaction with the ester group of the acrylic acid ester monomer, deprotonation at the a-position of the acrylic acid ester monomer and polymer, attack at the ester group of the polymer, or the like, each derived from the anion of the polymerization initiator and/or growing species.
The present inventors also tried to apply some of the anionic polymerization processes for a methacrylic acid ester as described in (3) to (6) to the preparation of a block copolymer having a polymer block of a methacrylic acid ester and a polymer block of an acrylic acid ester (particularly, a primary alkyl acrylate), however, the results were undesirable. The formation of a polymer block composed of an acrylic acid ester was found to cause great difficulty and the formation of a polymer block composed of a primary alkyl acrylate was found to be particularly difficult. The reason is presumed to be as follows: even if the anionic polymerization of a methacrylic acid ester proceeds stoichiometrically and a methacrylic acid ester polymer having an active anionic end is formed, the subsequent addition of an acrylic acid ester to the system considerably lowers the block formation efficiency of the acrylic acid ester at the active anionic end owing to the side reaction with the ester group of the acrylic acid ester monomer, deprotonation at the o-position of the acrylic acid ester monomer and acrylic acid ester polymer portion and attack to the ester group of the acrylic acid ester polymer portion.